Dioxygen activation with Bis(arylimino)pyridine radical anion complexes of nickel

Complexes of redox-active ligands have received increasing attention, in part, because they have been shown to facilitate a variety of bond-forming and bond-breaking reactions, including the activation of small molecules. Unique attributes of redox-active ligands include that they influence the electronic properties of transition metal complexes and may participate in their multielectron redox chemistry. In this work, the utility of Ni complexes of redox-active bis(arylimino)pyridine ligands for the activation of dioxygen was explored. A series of bis(arylimino)pyridine complexes [Ni{2,6-(ArN=CMe)₂C₅H₃N}Cl₂](where Ar = 4-X-2,6-ⁱPr₂C₆H₂ and X = H, Cl, Br, NO₂, or N(CH₃)₂) were synthesized along with the deuterium-labeled complex [Ni{2,6-(ArN=CCD₃)₂C₅H₃N}Cl₂](where Ar = 2,6-ⁱPr₂C₆H₃).Characterization by X-ray crystallography of three complexes confirmed coordination of the Ni center by the three nitrogen donor atoms of the bis(arylimino)pyridine ligand and two chloro ligands. The structural analyses further revealed a distorted square pyramidal geometry of the Ni centers. The reduced complexes[Ni{2,6-(ArN=CMe)₂C₅H₃N}Cl](where Ar = 4-X-2,6-ⁱPr₂C₆H₂ and X = H, Cl, or Br) and [Ni{2,6-(ArN=CCD₃)₂C₅H₃N}Cl](where Ar = 2,6-ⁱPr₂C₆H₃) were synthesized by reaction of the corresponding dichloride complexes with a sodium amalgam. These are four-coordinate complexes with distorted square planar geometry and are best characterized as ligand-radical complexes of Ni^II as opposed to Ni^I complexes on the basis of X-ray crystallography and electron paramagnetic resonance spectroscopy of Ni{2,6-(2,6-ⁱPr₂C₆H₃N=CMe)₂C₅H₃N}Cl]. Reactions of the ligand-radical Ni^II complexes with dioxygen were studied by UV-Vis spectroscopy, and the overall course of the reaction was similar for all four complexes. The half-life of the reaction was not affected by deuterium labeling of the acetimino methyl groups but was affected by different substituents in the 4-position of the phenyl groups of the ligand. The reactions caused intraligand C-C bond cleavage with formation of a new C=O double bond, resulting in the complexes [Ni{6-(ArN=CMe)C₅H₃N-2-C(O)NAr}Cl] (where Ar = 4-X-2,6-ⁱPr₂C₆H₂ and X = H, Cl, or Br) and [Ni{6-(ArN=CCD₃)C₅H₃N-2-C(O)NAr}Cl](where Ar = 2,6-ⁱPr₂C₆H₃). The structures of the complexes [Ni{6-(ArN=CR)C₅H₃N-2-C(O)NAr}Cl] (where Ar = 2,6-ⁱPr₂C₆H₃ and R = CH₃ or CD₃) were determined by X-ray crystallography. For all four complexes, the carboxamidato group was identified by a strong band in the infrared spectra assignable to the CO stretching vibration. Furthermore, the new carboxamidato ligands were removed from the metal center and isolated in their neutral form as 6-(ArN=CMe)C₅H₃N-2-C(O)NHAr (where Ar = 4-X-2,6-ⁱPr₂C₆H₂ and X = H, Cl, or Br) and 6-(ArN=CCD₃)C₅H₃N-2-C(O)NHAr(where Ar = 2,6-ⁱPr₂C₆H₃). Taken together, these results indicate that the activation of dioxygen occurs at the ligand and without an overall oxidation state change at the Ni center. The observed ligand-centered oxygenation in these reactions contrasts with the prevailing metal-centered chemistry of redox-active bis(arylimino)pyridine complexes and demonstrates that they can be directly involved in the activation and conversion of a small molecule.